My 1990 S2 has been making a strange moaning sound for quite some time. Sound comes from the passenger side around the oil filter area. Usually only happens on deceleration and you can hear and feel a fluttering at the oil filter.

I changed the o-rings on the OPRV once but that did not fix it. Finally removed my oil cooler this past week and installed new seals along with a new oil thermostat. It did not look bad but I figured it was about the only thing that could cause the noise.

So far the result is no more moaning and the car is running much cooler. Keeping my fingers crossed that it is finally fixed. Thought I would post it up here just in case someone else has the same problem.

 

Thanks for posting. I've had that problem from time to time, and one time, during a drive from Seattle to Carmel and back with the AC running in hot weather it got really bad crusing around 3400 rpm... made a very, very noticable loud moaning sound. I slowed down a little and it went away, sped up and it came back. Looks like I might need a new oil thermostat too.